1. Field of the Invention
The invention relates to monitors for determination of the condition of operating valves, and more particularly to systems such as those which monitor external to the valve.
Check valves are used extensively within nuclear power plants for the purpose of controlling the direction of fluid flow. The function of a check valve is to permit flow in only one direction. When the flow stops or reverses direction, the check valve should close immediately in order to prevent backflow.
Check valve failures have led to water hammers, overpressurization of low-pressure systems, and damage to flow system components. Degradation and wear of check valve internal components have contributed substantially to the number and severity of check valve failures. Principal causes of internal component wear are vibrations and oscillations (instability) of check valve obturators under flow conditions that do not fully open the check valve. Obturator instability may be a result of misapplication (using oversized valves) and exacerbated by low flow conditions and/or upstream flow disturbances.
A recent Significant Operating Experience Report distributed by the Institute of Nuclear Power Operations (INPO) recommends that nuclear power plants establish a preventive maintenance program to ensure check valve reliability. The program should include periodic testing, surveillance monitoring, and disassembly and inspection. A nuclear utility is thus motivated to determine which check valves are operating in an unstable or degraded condition.
Therefore a need exists for a method and device for positively determining the position and motion of a check valve obturator during plant operation. Those valves which are identified as unstable or degraded can then be disassembled more frequently which should reduce the number of failures which result from internal component wear.
Furthermore, those valves which are in satisfactory condition need not be disassembled. Avoidance of unnecessary disassembly minimizes costs, the potential for damaging the valve during disassembly or reassembly, improper reassembly, spread of radioactive contamination and radiation exposure to personnel, outage durations, and the potential for improper system restoration.
2. Description of the Prior Art
Several developmental and/or commercially available check valve monitors, particularly those based on measurements of acoustic emission, ultrasonics, radiography, and magnetic flux are described and discussed in a recent U.S. Nuclear Regulatory Commission report. The report, NUREG/CR-4302, Volume 2, also ORNL 6193/V2, is available from National Technical Information Service, Springfield, Virginia 22161, and is incorporated herein by reference.